This project is designed to determine whether the neonate is at greater risk than the adult from ingestion of potentially toxic heavy metals because of more rapid and/or extensive absorption from the gastrointestinal tract. The absorption kinetics of inorganic salts of lead (203), cadmium (109) and mercury (203) in the neonate will be compared to those in newly weaned and adult animals. Rats will be used as the primary animal model; important observations made in rats will be tested in a second species, the guinea pig, to provide an indication of the validity of generalizing the findings to man. The absorption of tracer amounts of lead, cadmium and mercury wil be directly determined using the in vivo segment technique which permits measurement of rates of uptake into the intestinal mucosa, retention in the gut wall and systemic absorption. Where appropriate, the overall extent of absorption of these metals after gastric intubation will be estimated indirectly by a method entailing determination of radioactivity remaining in the gastrointestinal tract. The effect of aspects of neonatal diet and gut physiology on absorption of these metals will be determined in the following two types of experiments; 1) the role of a largely milk diet in the neonate will be assessed by comparing the absorption of the metals in weaned control rats with an experimental group of the same age receiving only milk for one week, 2) the role of pinocytosis by the neonatal gut in the absorption process of these metals will be determined by comparing the absorption kinetics in controls to animals with markedly altered rates of pinocytotic activity resulting from adrenalectomy or glucocorticoid administration.